Groundwater monitoring sites, also referred to as water table monitoring wells or groundwater observation wells, serve the function of observing water tables and groundwater quality, particularly in the catchment area of water procurement plants. For constructing a groundwater monitoring site covering several aquifers at different depths, the first step involves drilling a borehole. On the basis of the geological profile of the borehole determined, the horizons for the individual monitoring points subject to observation are established. Subsequently, a string composed of individual pipes and carrying at its bottom end a filter pipe is installed for each monitoring point. Following installation of the strings, the borehole is packed with superposed layers of different materials. In the area of the monitoring points, the borehole is packed with filtering gravel. Layers of gravel filler and clay packing are conventionally provided between the individual monitoring points. In practice, major difficulties are frequently encountered in packing such known multiple monitoring points, because the position of the individual pipe strings within the borehole is only insufficiently controllable from above ground, particularly where boreholes of substantial depths are involved. An awkward position of the strings may result in insufficient packing of the borehole and in particular an insufficient sealing of the layers isolating the individual aquifers. This results in cross migration of waters corrupting the observation result.
To avoid these drawbacks, a pipe arrangement for monitoring the water table is already known (DE-PS 3,012,709) in which several pipes of different lengths interjoined close to their upper ends are installed in a steel casing which surrounds all pipes and is of a length approximately equal to the length of the longest pipe, in which the casing includes water inlet ports relatively spaced in a longitudinal direction in accordance with the position of the bottom ends of the pipes, the water inlet ports being coupled to the bottom end of the associated pipe through a manifold and being sealed against the interior of the casing. This arrangement facilitates the packing of the borehole annular chamber and the sealing of the layers isolating the individual aquifers. On the other hand, however, this pipe arrangement involves major constructional expenditure and is therefore only suited for small installation depths as they are sufficient for the observation of near-surface seepage waters. Further, the known pipe arrangement includes only small water inlet ports which do not permit sufficient quantities to be conveyed as they are necessary for withdrawing samples of groundwater.